Method of optically measuring black carbon in the atmosphere and apparatus for carrying out the method

ABSTRACT

For optically measuring black carbon in the atmosphere an aerosol particle collection area of a filter tape is continually illuminated by an illumination source with light of one or more wavelengths. Transmitted and reflected light fractions are measured at several precisely defined angles or angle ranges, such as of 0°, 12° to 140° and 165 to 180° by means of photodetectors arranged correspondingly relative to the illumination source, achieving maximum symmetry for the angles to be measured. The loading of the filter tape collection area with light absorbing aerosol material is continually determined from the change in the optical properties of the collection area with the aid of known algorithms from transmissivities and reflectivities as detected.

FIELD OF THE INVENTION

[0001] The invention relates to a method of optically measuring blackcarbon in the atmosphere and an apparatus for carrying out the method.

PRIOR ART

[0002] Black carbon (soot) is a leading component in particulateemissions from an incomplete combustion process. The German Standardhitherto for measuring black carbon in the air is VDI 2465 Sheet ½. Theguideline requirements define thermal methods as a reference forimmission measurements of black carbon, with the aid of which daily orweekly filter data is analyzed. This, however, fails to make it possibleto furnish the measuring data resolved in time/on a daily basis.

[0003] For optically measuring black carbon the ambient aerosol isdeposited on a single filter or filter tape. The change in the opticalproperties of the particle-loaded filter matrix as compared to aparticle-free matrix is determined either in transmission (ATN) or inreflexion (REF). The change in the blackening of the filter due toparticle loading is an indication of the mass loading of absorbingmaterial by application of a simple relationship derived from theLambert-Beer law $\begin{matrix}{{ATN} = {{{- 100}\ln \frac{T}{T_{0}}} = {\sigma_{ATN}S_{BC}}}} & \left( {1a} \right)\end{matrix}$

$\begin{matrix}{{REF} = {{{- 100}\ln \frac{R}{R_{0}}} = {2\quad \sigma_{REF}S_{BC}}}} & \left( {1b} \right)\end{matrix}$

[0004] where T, T₀ and R, R₀ stand for the transmittance and reflectancerespectively of the filter matrix as particle-loaded (no index) andparticle-free (index 0). S_(BC) designates the loading of the filterwith black carbon specific to the surface area of the collection area(in μgcm⁻²), σ_(ATN) and σ_(REF) are the proportionality factors betweenfilter loading S_(BC) and the attenuation of the light due toparticle-loading in transmitted light (σ_(ATN)) and in reflected light(σ_(REF)). Multiple scattering effects in aerosol or between particlesand filter matrix are neglected.

[0005] U.S. Pat. No. 4,893,934 describes an aethalometer comprising alight source and a sole light detector as well as two light paths fromthe light source to the light detector. A quartz fibre filter is alsoprovided whose collection area is located in the one light path, whilstthe other light path serves as the reference area. Through thecollection area of the filter ambient air is directed so that aerosolparticles are able to be deposited on the filter.

[0006] In the housing of the aethalometer a rotating disk is providedwith an opening so that light from the light source passes alternatelythrough the two light paths. The output voltage of the detector locatedbeneath the filter is applied to a voltage-controlled oscillator (VCO).The pulses for determining the light transmission passing separatelythrough the two light paths are counted and compared, from which theabsorption coefficient of the deposited aerosol particles is determined.

[0007] In the known aethalometer the particles are deposited on a filteror filter tape to thus permit determining the mass concentration of theblack carbon over a longer period of time, up to several monthsdepending on the concentration. Since in this known aethalometermultiple scattering effects as per equation (1a) are neglected, the massconcentration of the black carbon as established by the system employeddepends on the light-scattering aerosol components. Thiscross-sensitivity may result in the measured values being seriouslyfalsified. Apart from this, the air intake is unsuitable for largerparticle diameters.

[0008] For determining the black carbon content of atmospheric aerosolsamples an optical assembly was presented permitting simultaneousmeasurement of transmitted and reflected radiation (see: Petzold, A. andH. Kramer, “An improved aerosol absorption photometer for thedetermination of black carbon in ambient air”, Journal of AerosolScience, 32, pages 37-38, 2001). In this assembly aerosols are depositedon a filter tape and the collection area of the aerosol particles isilluminated with an LED. Photodetectors are arranged both in the areabetween the photodiode serving as the illumination source and the filtertape and beneath the filter tape so that both the transmitted lightfractions as well as the light fractions reflected by the filter tapecan be simultaneously measured.

SUMMARY OF THE INVENTION

[0009] The object of the invention is to improve the measurement andthus the determination of transmitted and reflected light fractions toachieve an enhanced signal average over an extended area.

[0010] In accordance with the present invention this and other objectscan be achieved by a method of optically measuring black carbon in theatmosphere, comprising the steps of depositing aerosols from a stream ofair onto a filter tape; illuminating an aerosol particle collection areaof said filter tape continually by an illumination source with light ofone or more wavelengths; measuring simultaneously light fractions bothtransmitted through and reflected from said filter tape at severalprecisely defined angles/angle ranges by means of photodetectorsarranged correspondingly relative to said illumination source inachieving maximum symmetry for the angles to be measured, anddetermining continually the loading of said filter tape collection areawith light-absorbing aerosol material from the change in the opticalproperties of said collection area caused by said loading with the aidof known algorithms from transmissivities and reflectivities asdetected.

[0011] In accordance with the invention for measuring the transmittedand reflected light fractions the photodetectors are arranged oppositeeach other at precisely defined angles or angle ranges of 0°, 120° to140° and from 165° to 180° preferably in ring-shaped mounting devicesarranged concentrically to the optical axis of the at least oneillumination source. The resulting maximum symmetry in the multipleangle measurement assembly in accordance with the invention for theangle ranges to be measured enhances a signal average over an expansivefilter area for a highly compact structure of a measuring apparatus inthe form of a measuring head.

[0012] To reduce scattered light the ring-shaped mounting devices inwhich the photodetectors are mounted opposite each other are preferablydefined in two different planes.

[0013] Instead of a reference path as provided in the knownaethalometer, for example, in accordance with the invention the lightintensity of the illumination source is continually measured byassigning a photodetector to the illumination source.

[0014] Furthermore, a dusting passage is configured in the measuringhead to ensure continual dusting of the filter tape and via which alsolarger particles (>10 μm) can gain access to the filter tape. Inaddition, for grading the size of the particles an externalpre-separator may be arranged upstream of the dusting passage.

[0015] When measuring with light of a single wavelength only anarrow-band light source, such as a color LED is employed. When makingthe measurement with several wavelengths a wideband light source is usedand a bandpass filter is arranged upstream of the individual detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will now be detailed with reference to the drawingsin which

[0017]FIG. 1 is a vertical section view of one embodiment of anapparatus in the form of a measuring head for a single wavelength;

[0018]FIG. 2 is a greatly simplified perspective representation of ameasuring assembly;

[0019]FIG. 3 is a diagram showing the angular distribution of radiationscattered in the two half-spaces as in dependence on the composition ofthe aerosol;

[0020]FIG. 4 is a graphic representation of the ratio of the signalsrelating to the various angles of observation θ;

[0021]FIG. 5 is a graphic representation of a signal ratio of detectorsat angles of observation θ=130° and 100° dependent on the fraction ofdiffuse scattering radiation, and

[0022]FIG. 6 is a graphic representation of the measured values ATN(transmission), REF (reflection) and ABS (multiple angle absorptionmeasurement) of a measured loading of the filter with black carbon asmeasured.

DETAILED DESCRIPTION

[0023] Referring to FIG. 1 there is illustrated diagrammatically anapparatus in the form of a measuring head 1. Provided in the upperhousing part 10 of the measuring head 1 is a LED 2 having a wavelengthof, for example, λ=670 nm as an illumination source. For monitoring thelight intensity a photodetector 3 is assigned to the LED 2. In themiddle portion of the upper housing part 10, two photodetectors 4 and,somewhat lower, two further photodetectors 5 are provided for measuringthe backscattered (reflected) radiation.

[0024] Provided between the upper housing part 10 and a lower housingpart 11 is a filter tape 6 indicated by the bold line. Arranged in thelower housing part 11 beneath the filter tape 6 is a furtherphotodetector 9 for measuring or detecting transmitted radiation.Furthermore, in the lower portion of the upper housing part 10 there isprovided a dusting passage 12 configured so that continual dusting ofthe filter tape 6 is assured.

[0025] Referring now to FIG. 2 there is illustrated in a greatlysimplified perspective representation how four photodetectors 4 ₁ to 4 ₄are provided on a first ring-shaped mounting device 7 ₁ preferablyequispaced angularly from each other below the illumination source inthe form of the LED 2 for detecting the backscatted radiation whilstsomewhat further down on a second ring-shaped mounting device 7 ₂likewise four photodetectors 5 ₁ to 5 ₄ are provided.

[0026] An aerosol particle-laden collection area 8 on the filter tape 6is evident from FIG. 2. Beneath the filter tape 6 there is provided thephotodetector 9 for transmitted radiation. In the perspectiverepresentation, the angles relating to the reflected radiation, i.e. theangles θ=0°; θ₁=130° and θ₂ =165°l are entered.

[0027] Arranged opposite each other in the two ring-shaped mountingdevices 7 ₁ and 7 ₂ in all cases are two photodetectors, for example, inthe first ring-shaped mounting device 7 ₁, the photodetectors 4 ₁ and 4₃ respectively 4 ₂ and 4 ₄ as well as in the second ring-shaped mountingdevice 7 ₂ arranged lower down the photodetectors 5 ₁ and 5 ₃respectively 5 ₂ and 5 ₄. As evident from FIG. 2 the ring-shapedmounting devices 7 ₁ and 7 ₂ are arranged concentrically to the opticalaxis of the measuring head 1 indicated dot-dashed.

[0028] By reason of this arrangement of the photodetectors 4 and 5respectively each in the form of detectors arranged opposite each other,a maximum symmetry with respect to the angle measuring assembly for theangle ranges to be measured is achieved in regards to the illuminationsource in the form of LED 2. With a highly compact configuration of themeasuring head, a better signal average over the extended collectionarea 8 on the filter tape 6 is attained.

[0029] The optimum position of the photodetectors 4, 5 for detecting theradiation transmitted and reflected as a whole was derived from theanalysis of the angle distribution of the loaded filter. This analysisshowed that the angle distributions can be represented by a linearcombination of a fraction of a diffusely scattered radiation and afraction of radiation reflected at a rough surface. The parameterizedangle distributions can be represented as $\begin{matrix}{{{S(\theta)} = {I\quad \cos \quad {\theta \left( {{{for}\quad {the}\quad {front}\quad {half}\text{-}{space}},{\theta = {0 - 90^{{^\circ}}}}} \right)}}}{and}} & \left( {2a} \right) \\{{{S(\theta)} = {I\left( {{{\alpha cos}\left( {\theta - 180^{{^\circ}}} \right)} + {\left( {1 - \alpha} \right){\exp \left\lbrack {{- \frac{1}{2}}\frac{\left( {\theta - 180^{{^\circ}}} \right)^{2}}{\sigma^{2}}} \right\rbrack}}} \right)}}\left( {{{for}\quad {the}\quad {rear}\quad {half}\text{-}{space}},{\theta = {90 - 180^{{^\circ}}}}} \right)} & \left( {2b} \right)\end{matrix}$

[0030] where α is the fraction of the diffusely scattered radiation andσ the roughness of the filter surface (see also FIG. 3).

[0031] Referring to FIG. 3 there is illustrated the angle distributionof the radiation scattered in the front half-space, θ=0-90° and in therear half-space, θ=90-180° dependent on the composition of the aerosol.Hereby, the composition of the aerosol is entered as a fraction of thelight absorbing components of the black carbon (BC) in the mass as awhole. In this graph the scattering angle θ in degrees is entered on theabscissa and the normalized scattering intensity is entered on theordinate.

[0032] The positioning of a detector with θ₁=130° permits distinguishingdiffusely scattered radiation from reflected radiation with maximumresolution. (See FIG. 4) Entered on the left-hand axis/ordinate is theratio of the signals for angles of observation θ and θ=165° as afunction of the diffuse fraction of the scattered radiation and on theright-hand Y axis/ordinate the difference of the signal ratios fortotally diffuse and totally reflected radiation as a function of theangle of observation θ.

[0033] The relation between the measured signal ratio S(θ₁)/S(θ₂) andthe diffuse fraction of the backscattered radiation is linear. (See FIG.5) Entered in FIG. 5 is the signal ratio of the detectors at the anglesof observation 130° and 100° in relation to an angle of 165° independence on the fraction of diffuse scattering with the fraction a ofdiffuse scattering on the abscissa and the ratio S(θ)/S(165°) on theordinate.

[0034] This thus permits definitely obtaining the diffuse scatteredfraction α from the signal ratio as measured. Having determined theparameter α then permits in conclusion calculating the total radiationscattered in the rear half-space from equation (2b). To obtain thetransmitted radiation in the front half-space from equation (1) ameasurement at θ=0° suffices.

[0035] The total intensities as thus obtained are, for the fronthalf-space, $\begin{matrix}{I_{t} = {{\int_{- 90^{{^\circ}}}^{90^{{^\circ}}}{{S\left( {\theta = 0^{{^\circ}}} \right)}\cos \quad \theta \quad {\theta}}} = {2{S\left( {\theta = 0^{{^\circ}}} \right)}}}} & (3)\end{matrix}$

[0036] and, for the rear half-space, $\begin{matrix}\begin{matrix}{I_{t} = {{\alpha {\int_{90^{{^\circ}}}^{270^{{^\circ}}}{{S\left( {\theta = 180^{{^\circ}}} \right)}{\cos \left( {\theta - 180^{{^\circ}}} \right)}\quad {\theta}}}} + {\left( {1 - \alpha} \right){\int_{90^{{^\circ}}}^{270^{{^\circ}}}{S\left( {\theta = 180^{{^\circ}}} \right)}}}}} \\{{{\exp \left\lbrack {{- \frac{1}{2}}\frac{\left( {\theta - 180^{{^\circ}}} \right)^{2}}{\sigma^{2}}} \right\rbrack}{\theta}}} \\{= {{S\left( {\theta = 180^{{^\circ}}} \right)}\left( {{2\alpha} + {\left( {1 - \alpha} \right)\sqrt{2\pi}\sigma}} \right)}}\end{matrix} & (4)\end{matrix}$

[0037] From these radiation intensities, the light absorption caused bythe deposited particles is determined via a known algorithm (see HanelG., Radiation budget of the boundary layer, Part II, Simultaneousmeasurement of mean solar volume absorption and extinction coefficientof particles, Phys. Atmosph., 60, 241-247, 1987).

[0038] As a result, this algorithm furnishes the optical density τ_(L)of the filter loaded with the particles and the ratio SSA_(L) of thelight scattering to light extinction (single scattering albedo) of theloaded filter. It is from these parameters that in conclusion the massloading of the filter with light-absorbing aerosol S_(BC) is determinedvia

ABS=100 (1−SSA _(L)) τ_(L)=σ_(ABS) S _(BC)  (5)

[0039] The parameter GABS can be obtained from calibrating the methodagainst a chemical method of measuring the black carbon (e.g. VDI 2465,Part 1) in the aerosol.

[0040] Application Example

[0041] The described method was put to use in determining the blackcarbon content in a mixture of light-scattering carbon). The masspercentage of the black carbon varied between 1% and 100%. In an idealmethod the change in the optical filter properties (transmissivity,equation (1a), reflectivity, equation (1b), absorptance, equation (5))caused by the particle loading of the filter is directly proportional tothe loading of the filter with black carbon and is thus represented byan originating straight line.

[0042]FIG. 6 shows the relationship between the measured values ATN, REFand ABS and the loading of the filter with black carbon as measuredindependently in accordance with VDI 2465, Sheet 1, i.e. for atransmission measurement (ATN), reflectivity measurement (REF) and forthe method as described (ABS).

[0043] Table 1 lists the corresponding results of the correlationanalysis. The multiple angle absorption measurement as described abovethus exhibits high correlation for simultaneously zero crossover of theregression straight lines. Current prior art methods exhibit either astrong scattering in the measured values (transmission) or an interceptin a significant departure from zero (reflectivity). This thus documentsthe improvement, as anticipated, in determining the black carbon in theair by the multiple angle absorption measurement as described.

[0044] A correlation analysis of the relation between the measured valueof the transmission measurement (ATN), reflectivity measurement (REF)and multiple angle absorption method (ABS) as well as the loading of thefilter with black carbon as measured in accordance with VDI 2465, Sheet1 is given in the following Table 1. TABLE 1 ATN REF ABS n 28 28 28 r² 0.62  0.89  0.91 intercept  0 17.3 ± 4.5  0 slope 7.2 ± 0.5  2.8 ± 0.23.3 ± 0.1

[0045] Further fields for industrial application are: continual blackcarbon mass concentration monitoring in the immission in environmentnetworks, measuring black carbon emission in combustion processes(automotive engines, aircraft engines, firing systems),

[0046] workplace monitoring, for example in factory buildings, on truckloading ramps, wharves;

[0047] ventilation monitoring, for example in factory buildings or intunnel monitoring.

What is claimed is:
 1. A method of optically measuring black carbon inthe atmosphere, comprising the steps of: depositing aerosols from astream of air onto a filter tape, illuminating an aerosol particlecollection area of said filter tape continually by an illuminationsource with light of one or more wavelengths, measuring simultaneouslylight fractions both transmitted through and reflected from said filtertape at several precisely defined angles/angle ranges by means ofphotodetectors arranged correspondingly relative to said illuminationsource in achieving maximum symmetry for the angles to be measured, anddetermining continually the loading of said filter tape collection areawith light-absorbing aerosol material from the change in the opticalproperties of said collection area caused by said loading with the aidof known algorithms from transmissivities and reflectivities asdetected.
 2. The method as set forth in claim 1 wherein said transmittedand reflected light fractions are measured at angles/angle ranges of 0°,120 to 140° and 165 to 180° and then averaged.
 3. The method as setforth in claim 1 or 2 wherein in measurement at a single wavelength onlya narrow-band light source is employed.
 4. The method as set forth inclaim 3 wherein a color LED is used as said narrow-band light source. 5.The method as set forth in claim 1 or 2 wherein in measurement atseveral wavelengths use is made of a wideband light source and bandpassfilters are employed upstream of said individual light detectors.
 6. Themethod as set forth in claim 1 wherein the light intensity of saidillumination source is measured and determined continually.
 7. Anapparatus for implementing the method as set forth in any of the claims1 to 6 in the form of a measuring head (1) comprising a illuminationsource (2) arranged above a filter tape (6), a photodetector (7) beneathsaid filter tape (6) for measuring transmitted light fractions andphotodetectors arranged between said illumination source (2) and saidfilter tape (6) for measuring reflected light fractions wherein, in eachcase, at least two of said photodetectors arranged between saidillumination source (2) and filter tape (6) are provided opposite eachother in ring-shaped mounting devices (7 ₁, 7 ₂) relative to saidoptical axis of said illumination source (2) and are oriented atprecisely defined angles/angle ranges θ of 0°, 120 to 140° and 165 to180° relative to said filter tape surface, and units (20, 21) fordetermining the loading of said filter tape with light-absorbingmaterial are arranged downstream to said photodetector beneath saidfilter tape and said photodetectors arranged opposite each other.
 8. Theapparatus as set forth in claim 7 wherein to reduce scattered light saidphotodetectors provided opposite each other are accommodated in tworing-shaped mounting devices (7 ₁, 7 ₂) located at two different planes.9. The apparatus as set forth in claim 7 or 8 wherein a LED (3) isprovided for monitoring said light intensity of said illumination source(2).
 10. The apparatus as set forth in claim 7 wherein in said measuringhead (1) a dusting passage (12) is configured so that in addition tocontinual dusting of said filter tape (6) also coarser particles (>10μm) gain access to said filter tape.
 11. The apparatus as set forth inclaim 10 wherein a preseparator is provided upstream of said dustingpassage (12) for size selection of said particles.